Dealer Won't Care if It Is Raining or Shining
You may think that a rainy day will be the best day to go for a car. You may also think you'll be the only customer in a dealership, and salesmen will be more eager to negotiate an upcoming car deal. However, they will think about quite the opposite thing; if you are here on a rainy day like this, you need a car urgently and that's it.
You can just get drenched instead of cutting a car deal. Such guys like us - car dealers - won't care much if it is raining or shining; our negotiating behavior won't change. Besides, there is another important thing if you are shopping for a used car - you can fail to properly inspect the condition of it in such weather. So, I'd advise you not to use such a day as a sort of helpful tool in such a matter like car purchase because that is certainly not the best time for buying a car.